The invention relates generally to flow sensors, and more particularly, to variable orifice fluid flow sensors.
Orifice flow sensors are used to measure the flow rates of fluids, which include liquids and gases. A typical orifice flow sensor comprises a fixed orifice through which a fluid is made to flow. A pressure difference is established between the fluid that is present upstream from the orifice and the fluid that is flowing through the orifice. This pressure difference can be used to measure the flow rate of the fluid. For this purpose, a pressure transducer measures the pressure difference that is established across the orifice, and is calibrated such that the flow rate of the fluid is calculated from this pressure difference.
Variable orifice flow sensors provide sufficient pressure difference for measurement purposes across a broad range of flow rates. This is achieved by introducing a bending member into the fluid flow passage. The bending member is mounted to the housing for the fluid flow passage and includes a flapper that is positioned across the fluid flow passage and bends or flexes in the direction of the fluid flow as a result of contact with the fluid flow, and hence creates a variable orifice within the fluid flow passage. The measurement of flow rates in a variable orifice flow sensor is similar to the measurement of flow rates in fixed orifice flow sensors. That is, a pressure transducer measures the pressure difference across the variable orifice and calculates the flow rate of the fluid from the pressure difference.
U.S. Pat. Nos. 4,989,456; 5,033,312; 5,038,621; 6,722,211 and 7,270,143 show variable orifice flow sensors.
The performance of the sensor can be directly influenced by the connection of the bending member including the flapper to the housing that defines the fluid flow passage. In situations where the bending member is rigidly secured to the housing, this tight engagement with the housing can distort the movement of the flapper to negatively affect its operation. Further, in situations where the bending member is too loosely secured to the housing, it is possible for fluid to flow around the bending member through leaks located between the housing and the bending member.
In either situation, the movement and operation of the flapper is affected by the connection of the bending member to the housing, and hence the measured pressure difference across the variable orifice defined by the flapper becomes altered, such as by poor low flow resolution and non-linear movement of the flapper. This, in turn, leads to inaccurate measurements of the flow rate of the fluid.